The present invention relates to a system and method for determining a parameter of a solid object or workpiece such as a semiconductor wafer, and more particularly relates to a system and method for determining, in real time the emissivity and/or temperature of the wafer during thermal processing.
Thermal processing furnaces have been widely known and used for many years to perform a variety of semiconductor fabrication processes, including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products. Thermal processing furnaces typically employ either a horizontal-type furnace or a vertical-type furnace. For some applications, vertical-type furnaces are preferred because they create less particles during use, thus decreasing the incidence of contamination and wafer waste, they can be easily automated, and they require less floor space because of their relatively small footprint.
Both conventional types of furnaces are designed to heat semiconductor wafers to desired temperatures to promote either diffusion of implanted dopants to a desired depth while maintaining line width smaller than 1 micron, as known, or to perform other conventional processing techniques, such as the application of an oxide layer to the wafer or deposition of a chemical vapor layer to the wafer. The heating requirements of the wafer during processing are known and well understood, and thus are closely monitored.
Conventional vertical-type thermal processing furnaces, such as tube furnaces, are designed to support the processing tube within the furnace in the vertical position. The thermal furnace also typically employs a wafer boat assembly which is mounted to appropriate translation mechanisms for moving the wafer boat into and out of the processing tube. A wafer-handling assembly is deployed adjacent and parallel to the wafer-boat assembly to transfer the semiconductor wafers from wafer cassettes to the wafer-boat assembly. The wafers are then raised into a quartz or silicon heating tube. The tube is then slowly raised to the desired temperature and maintained at that temperature for some pre-determined period of time. Afterwards, the tube is then slowly cooled, and the wafers removed from the tube to complete the processing. A drawback of this processing technique is that it places constraints on the time-at-temperature to which a wafer can be subjected. Conventional vertical furnaces of these and other types are shown and described in U.S. Pat. No. 5,217,501 of Fuse et al. and in U.S. Pat. No. 5,387,265 of Kakizaki et al.
As the critical dimensions for silicon integrated circuits are continuously scaled downward into the sub-micron regimes requirements for within wafer temperature uniformity and wafer-to-wafer temperature repeatability become more stringent. For example, in 0.18 .mu.m technology, the required wafer-to-wafer temperature repeatability is in the order of +/-3.degree. C.
Pyrometry has been one method of choice for non-contact temperature measurements of a silicon wafer during processing in a thermal processing furnace, but it suffers from known drawbacks. One drawback is that the emissivity of the wafer backside must be known in order to attain accurate temperature measurements. Typically, silicon wafers have backside layers that can drastically alter the spectral emissivity of the wafer through interference effects, which can lead to temperature measurement errors during processing. Furthermore, the emissivity of the wafer is also dependent on the backside surface roughness and wafer temperature. All of these drawbacks make the determination or prediction of wafer emissivity a difficult task.
Prior art techniques have attempted to measure the wafer emissivity in situ, that is, within the furnace or heating chamber, in order to measure the temperature of the wafer during processing. One prior art method for determining wafer emissivity is to employ an AC ripple technique, as set forth, for instance, in U.S. Pat. No. 5,310,260. A light source is employed to illuminate the wafer backside within a heating chamber of the thermal processing apparatus. The radiation reflected from the wafer and the source intensity are measured, and the magnitude of the AC components of the source are extracted. The wafer emissivity is then calculated using a ripple equation. A drawback of this approach is that it occurs completely within the heating or process chamber of the thermal processing furnace, and hence it is difficult, if not impossible, to hemispherically and uniformly illuminate the wafer therein. Consequently, it is difficult to accurately determine wafer emissivity, especially in real-time, during processing.
Another drawback of prior art systems is that the heating lamps used to heat the chamber and thus the wafer are also employed to illuminate the wafer. Further, the orientation and position of the heat lamp is fixed in the system. This fixed lamp position makes it difficult to hemispherically and uniformly illuminate the wafer when disposed within the heating chamber. Moreover, the AC ripple generated by the heat lamps is used to determine wafer reflectivity. The combination of the fixed lamp position and the AC ripple often results in inaccurate wafer reflectivity measurements.
Another difficulty associated with determining wafer emissivity and hence temperature during processing is accurately determining the radiation flux within the chamber during processing. This problem arises since stray light, that is, radiation from sources other than the wafer, is reflected onto the pyrometer when measuring the radiation flux within the chamber. This measured radiation value is aggregated with the radiation emitted by the wafer, and employed to determine the wafer temperature. Since the wafer emittance is all that is desired, the pyrometer signal does not accurately measure radiation emitted just from the wafer. Conventional systems cannot accurately and completely compensate for this stray light component, and hence have difficulty achieving the temperature accuracy required by modern manufacturing techniques.
Due to the foregoing and other shortcomings of prior art thermal processing furnaces, an object of the present invention is to provide a system for accurately determining, in real time, the wafer emissivity.
Another object of the invention is to provide a system for measuring and correcting for stray light within the process chamber.
Other general and more specific objects of the invention will in part be obvious and will in part appear from the drawings and description which follow.